1. Field of the Invention
The present invention relates to methods and apparatus for metrology usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
The targets used by conventional scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). This simplifies reconstruction of the target as it can be regarded as infinite. However, in order to reduce the size of the targets, e.g., to 10 μm by 10 μm or less, e.g., so they can be positioned in amongst product features, rather than in the scribe lane, so-called dark field metrology has been proposed. Placing the target in amongst the product features increases accuracy of measurement because the smaller target is affected by process variations in a more similar way to the product features and because less interpolation may be needed to determine the effect of a process variation at the actual feature site.
In dark field metrology, a small grating is illuminated with a large measurement spot (i.e., the grating is overfilled). Often the illumination is off-axis, in other words, the measurement beam is incident on the target within a narrow angular range not including the normal to the substrate. In the measurement branch of the scatterometer, the zeroth order diffracted by the target is blocked by a field stop and the grating is imaged on a detector using only one of the first order diffracted beams. A second image of the grating is obtained using only the other first order diffracted beam. If the grating is formed by superimposition of identical gratings, a measure of the overlay error between those two superimposed gratings can be obtained from the difference in intensity of the images formed from the respective first order diffracted beams. Dark field metrology has some disadvantages. For example, throughput is reduced as two images of each grating are required and multiple pairs of differently biased gratings are required for accurate determination for overlay. The use of multiple pairs of gratings also increases the space on the substrate that needs to be devoted to metrology targets and hence is unavailable for product features. Because the gratings are overfilled, aberrations in the imaging system can cause variations in the position of the grating to influence the measurement results.